The idea of writing the query letter early in this process is that it will help you get focused. I assume that your focus will shift, as you do more research, but it’s good to get to someplace fairly quickly. When you revise your query later in the semester, I expect it will be quite a different letter.
A query letter does a few simple things. Most importantly, in a query letter you are inquiring into whether or not a magazine or journal (or, in other cases, a book publisher) might be interested in considering an article for publication. You do this by giving them a taste of the article. Many queries actually excerpt a small bit—maybe a paragraph—from the article itself. In any case, you need to describe the substance of your article—even if it is still just an idea.
You also use a query letter to introduce yourself and your qualifications as the author(s) of the piece. But probably the most important thing to accomplish in a query letter is to articulate why your article idea is a good fit for their publication. Having a good article idea is fine and good, but it won’t necessarily get you published; you need to have a good idea that fits the audience of a particular publication. You accomplish this mostly by describing your article in a way that is familiar to the editor of the magazine. The old adage of “show, don’t tell” applies here, I think; you don’t want to say things like, “Because your readers are interested in alternative fuels…” Instead, you just describe an article that sounds like the kind of thing they would publish, using specific examples. You could also point out similar articles they’ve published or, even, gaps in their coverage of topic X (being careful not to sound critical). Professional communication is tricky.
Query letters, like most professional correspondence, should be single spaced; its tone and convention-use should also be professional (that is, not overly informal). In many cases now, a query will happen by email, but I’d like you to treat this assignment as if it is going be sent by the USPS.